This invention relates to the production of chlorine and sodium hydroxide (caustic soda) from sodium chloride brine employing an electrolytic permselective membrane cell. More particularly, this invention relates to an improved method of opertion of a permselective membrane cell.
As is known in the prior art, a permselective membrane cell consists of three basic elements; anode, membrane, and cathode. The anode and cathode are each contained in compartments separated from one another by the membrane. The assembly of these components constitutes a unit-cell. An electrolyzer can be made up from a number of unit-cells assembled together in a stack. If the anode of one unit-cell is connected electrically to the cathode of the adjacent unit-cell, the electrolyzer is said to be bipolar, and if all anodes are connected together electrically, and all cathodes connected similarly, the electrolyzer is said to be monopolar.
In a permselective membrane cell, it is desirable to operate with a relatively narrow gap between the two electrodes so as to minimize the voltage drop imposed by the electrical resistance of the electrolyte. The total gap is made up of an anolyte gap and a catholyte gap. The relative size of each gap is, of course, dictated by the location of the membrane.
Cationic permselective membranes of the type usually employed in chlorine/caustic soda cells, typically perfluorosulfonic acid-type membranes with equivalent weights ranging from 900 to 1200, are vulnerable, as a result of the voltage gradient, to a certain amount of back-migration of hydroxyl ions from the cathode compartment to the anode compartment. With the evolution of chlorine from the anode, this results in formation of a relatively high local concentration of alkaline hypochlorite in the immediate vicinity of the anode side of the membrane. The conductive coating used on the titanium metal anodes employed in chlorine/caustic soda membrane cells is most commonly a mixture of ruthenium and titanium oxides. Such coatings are susceptible to attack by alkaline hypochlorite, leading to rapid loss of coating with the result that the anode surface becomes non-conductive. As a consequence of this susceptibility, it is necessary to prevent the membrane from coming into direct contact with the anode. Hence, this dictates that the membrane be located close to the cathode and away from the anode. A method heretofore employed in chlorine/caustic soda membrane cells to retain the membrane in a fixed location involves a spacer or separator grid between the anode and the membrane thus preventing direct contact between anode and membrane. A similar spacer may be employed between the cathode and membrane if it is desired to prevent the membrane from coming into direct contact with the cathode.
There are two major disadvantages of this method; first, the spacer blocks a portion of the membrane, thus restricting the flow of sodium ions through the membrane and consequently increasing the voltage drop across the cell. Second, the spacer interferes with the release of gas, chlorine on the anode side and hydrogen on the cathode side, from the immediate vicinity of the electrodes, thus interfering with the flow of electrical current through the electrolyte and contributing further to an increased voltage drop across the cell.
Thus, it would be advantageous to operate a membrane cell without the presence of spacers, yet still maintain the membrane in a fixed location close to, or even in direct contact with, the cathode while preventing substantial contact of the membrane and the anode.